1. Field of the Invention
The present invention relates to an anomaly judgment apparatus for a secondary air supply system, which is adapted to supply secondary air to a portion of an exhaust path of an internal combustion engine located upstream of a catalytic converter disposed in the exhaust path.
2. Description of the Related Art
Conventionally, in order to eliminate harmful components (CO, HC, and NOx) from exhaust gas emitted from an internal combustion engine, a three-way catalytic converter (herein may be referred to merely as a “catalytic converter”) is disposed in an exhaust path of the internal combustion engine. When the temperature of the three-way catalytic converter is lower than an activation temperature needed for the three-way catalytic converter to exhibit its catalytic function, the three-way catalytic converter fails to sufficiently exhibit its exhaust purification function. Therefore, when the temperature of the three-way catalytic converter is lower than the activation temperature; for example, in the case where an internal combustion engine is started when the temperature of the internal combustion engine is near ambient temperature (outside air temperature) (hereinafter referred to as “at the time of cold start”), the temperature of the three-way catalytic converter must be increased as soon as possible so as to activate (warm up) the three-way catalytic converter.
In order to fulfill the above need, a secondary air supply system is widely used. The secondary air supply system supplies secondary air to a portion of the exhaust path located upstream of the three-way catalytic converter, whereby the three-way catalytic converter is actively warmed up by heat of reaction that is generated when oxygen contained in the secondary air oxidizes unburnt components (particularly HC) contained in exhaust gas.
Component parts of such a secondary air supply system generally include an air pump and an air switching valve. The air pump is adapted to introduce air into a secondary air supply path connected to a portion of the exhaust path located upstream of the three-way catalytic converter. The air switching valve is provided in a portion of the secondary air supply path located downstream of the air pump and adapted to control opening and closing of the secondary air supply path. When anomaly arises in such a component part of the secondary air supply system, the following problem arises. Even when an instruction to supply secondary air is issued to the secondary air supply system, secondary air is not supplied, and thus warming up of the three-way catalytic converter is delayed, resulting in an increase in the quantity of emissions. Alternatively, even when an instruction to stop supplying secondary air is issued to the secondary air supply system, secondary air continues being supplied, and thus the air-fuel ratio of exhaust gas shifts to the lean side, resulting in a drop in NOx elimination efficiency. Therefore, when anomaly arises in the secondary air supply system (component parts of the same), the occurrence of the anomaly must be detected.
In view of the above, a secondary air supply system (anomaly judgment apparatus for the same) disclosed in Japanese Patent Application Laid-Open (kokai) No. 9-21312 is configured as follows. A pressure sensor is provided to detect pressure in a portion of a secondary air supply path that extends between an air pump and an air switching valve, which are component parts of the secondary air supply system. Whether or not any of the component parts is anomalous is judged on the basis of the results of judgment as to whether the pressure detected by the pressure sensor falls within a predetermined normal range.
In the above-disclosed secondary air supply system, a so-called “normally-closed negative-pressure-responsive-type open-close valve” is used as the air switching valve. The negative-pressure-responsive-type open-close valve utilizes, as negative pressure, the pressure in a portion of an intake path downstream of a throttle valve (hereinafter referred to as “throttle valve downstream pressure”), and is opened by drive force produced by the negative pressure. Accordingly, the secondary air supply system is configured such that when the secondary air supply system is instructed to supply secondary air, the system introduces the throttle valve downstream pressure (according, negative pressure) to the air switching valve, to thereby open the air switching valve. Accordingly, in order to reliably open the air switching valve upon receipt of an issued instruction for secondary air supply, a sufficient degree of negative pressure must be secured (i.e., the throttle valve downstream pressure must be a sufficiently low, stable pressure) at the time when the instruction is issued.
Incidentally, the throttle valve downstream pressure changes depending on operating conditions of the engine. Further, a surge tank is generally disposed in the intake path to be located downstream of the throttle valve in order to increase the volume of the intake path for the purpose of, for example, attenuating intake air pulsation of the engine. Therefore, the throttle valve downstream pressure has characteristics such that once the pressure becomes high, the pressure does not immediately decrease to a sufficiently low pressure.
As can be understood from the above, if the throttle valve downstream pressure is so high that the above-described sufficient level of negative pressure cannot be secured at the time of issuance of an instruction for secondary air supply, there may continue a state in which although the air switching valve is normal, the air switching valve is not reliably opened over a certain period of time after issuance of the instruction. Therefore, if judgment as to whether or not the secondary air supply system (component parts of the same) is anomalous is performed by use of the above disclosed anomaly judgment apparatus in the above-described state, the pressure detected by the pressure sensor falls outside the predetermined normal range, whereby the secondary air supply system (the air switching valve, in this example), may be erroneously judged to be anomalous, even if the secondary air supply system is normal.